“Bi-Functional” Electrode Materials for Na-Ion Batteries

ECS Electrochemistry Letters, 3 (4) A23-A25 (2014)
2162-8726/2014/3(4)/A23/3/$31.00 © The Electrochemical Society
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Na2/3 Ni1/3 Ti2/3 O2 : “Bi-Functional” Electrode Materials
for Na-Ion Batteries
Rengarajan Shanmugam and Wei Laiz
Department of Chemical Engineering and Materials Science, Michigan State University, East Lansing,
Michigan 48824, USA
In this paper, we demonstrate the electrochemical properties of a P2-type layered oxide, Nax (Ni2+ )1/3 (Ti4+ )2/3 O2 (x = 2/3), as
“bi-functional” electrode material for room temperature, non-aqueous Na-ion batteries. Making use of the high-voltage redox couple
Ni2+ /Ni3+ or the low-voltage redox couple Ti4+ /Ti3+ , we substantiate Na2/3 Ni1/3 Ti2/3 O2 can function either as a cathode with
an average voltage of 3.7 V and 75 mAh/g at C/20 or a anode with an average voltage of 0.7 V and 75 mAh/g at C/20. The
cathodic Na2/3 Ni1/3 Ti2/3 O2 displays reversible sodium insertion/extraction but has a lower rate capability compared with anodic
Na2/3 Ni1/3 Ti2/3 O2 .
© 2014 The Electrochemical Society. [DOI: 10.1149/2.007404eel] All rights reserved.
Manuscript submitted November 5, 2013; revised manuscript received January 24, 2014. Published February 6, 2014.
Geographical limitations and uncertainties about the availability
of sufficient lithium resources have led researchers to develop alternative battery chemistries, such as Na-ion batteries, based on cheaper
and more widely available elements, especially for bulk energy storage applications.1–6 Developing good electrode materials that can reversibly host sodium ions is crucial for further technological advancement and commercialization of Na-ion batteries.
One of the prototypic sodium electrode materials is Nax CoO2
(x∼0.7)7 with a hexagonal layered structure (space group P63 /mmc),
also denoted as P2-type by Delmas et al.8 Na0.7 CoO2 was able to
sustain sodium insertion/extraction at a potential range of 2–3.8 V.9
However, this material exhibited complex phase transformation in this
window, possibly due to the interplay of Na+ /vacancy ordering at Na
sites and charge ordering at Co sites.10,11 The application of two or
more transition metals with different valences is likely to interfere
with vacancy/charge ordering and lead a more solid-solution behavior. Good electrochemical behaviors were reported for mixed valence
P2-type oxides such as Nax Ni1/3 Mn2/3 O2 ,12,13 Nax Fe1/2 Mn1/2 O2 ,14
Na0.45 Ni0.22 Co0.11 Mn0.66 O2 ,15 Na0.85 (Li0.17 Ni0.21 Mn0.64 )O2 ,16 etc, although some plateaus/steps were still visible suggesting phase transformation. Na3 V2 (PO4 )3 has been shown to work as “bi-functional”
electrode using V3+/4+ (Eo = 3.40 V) and V2+/3+ (Eo = 1.63 V)
electro-active redox couples.17 However, the toxicity of vanadium
can potentially become an issue for building large-scale, commercial
devices.
The excellent ionic conductivity of another mixed valence P2type material Nax (Ni2+ )1/3 (Ti4+ )2/3 O2 (x = 2/3), was reported
previously.18,19 By selectively activating the high-voltage redox couple Ni2+ /Ni3+ or the low-voltage redox couple Ti4+ /Ti3+ , we suppose
it can function either as a cathode with 90 mAh/g (x from 2/3 to 1/3) or
as an anode with the same 90 mAh/g (x from 2/3 to 1). In this letter, we
report for the first time electrochemical properties of Na2/3 Ni1/3 Ti2/3 O2
(SNTL), as an alternative bi-functional electrode either at the high or
low voltage windows using two different redox active species in the
same material.
Experimental
Starting materials (all from Sigma-Aldrich) for the synthesis of
Na2/3 Ni1/3 Ti2/3 O2 (SNTL) powders were stoichiometric amount of
Na2 CO3 (≥99.5%), NiO (micron powder, 99%; < 50 nm powder,
99.8%), and TiO2 (micron powder, 99%; 21 nm powder, ≥99.5%).
For a typical synthesis of 10 g SNTL powder, 4.6 g of Na2 CO3 ,
2.5 g of NiO, and 5.4 g of TiO2 were used. Na2 CO3 of 10 wt%
excess were added to compensate the sodium oxide evaporation during
high-temperature processing. Both micron- and nano-sized NiO and
TiO2 were employed to compare the effect of particle size on the
ease of synthesis and the electrochemical properties. Micron-sized
z
E-mail: [email protected]
precursors were subjected to dry-milling using SPEX SamplePrep
8000 M mixer/mill. Nano-sized precursors were mixed in a jar mill
with 2-propanol as solvent. After uniform mixing and solvent removal
at 120◦ C, the powders were fired at 900◦ C in air within a box furnace.
The synthesized powders were made into a slurry which contained 80 wt% active powders, 10 wt% PVDF binder in N-methyl2-Pyrrolidone (NMP), and 10 wt% Timcal Super-P as conductive
additives. Powders from nano-sized precursors were used unless stated
otherwise. The slurry was cast into a composite film on an aluminum foil. The loading of active material was around 4 mg/cm2 .
The Swagelok cell assembly was carried out inside the glove box
with sodium metal serving as counter and reference electrodes. 0.5 M
NaPF6 in 50/50 vol% of ethylene carbonate (EC)/diethyl carbonate
(DEC) was used as the electrolyte for testing. Electrochemical testing
was performed with a Bio-logic VSP300 workstation. Galvanostatic
charging was carried out in a voltage window of 2.0–4.2/4.5 V for
the cathode and 0.2–3.0 V for the anode. The cell was disassembled
inside the glove box and the film was washed with dimethyl carbonate
solvent to remove residual salts and eventually the solvents were dried
in the glove box.
Powder X-ray diffraction (XRD) measurements were performed
using a Bruker D8 ADVANCE diffractometer using Cu-Kα X-rays.
Both pristine powders and composite films were studied. The powder
samples were also imaged in a scanning electron microscope (JEOLJSM 7500-F). The BET surface area of the powders was measured
using Micromeritics ASAP 2020 instrument with Krypton as the adsorbate gas.
Results and Discussion
Powder characterization.— It was found that micron-sized precursors yielded phase-pure samples at 900◦ C when fired for 12 hours
while nano-precursors yielded pure powders in just 2 hours, as observed in XRD in Figure 1a. Below 2 hours, impurity peak probably
due to NiO appeared. Micron-sized and nano-precursor powders were
5–10 μm and less than 5 μm, respectively, as evident from SEM
micrographs in Figure 1b. The powders made from micron-sized precursors and nano-precursors had a BET surface area of 0.58 m2 /g and
1.30 m2 /g, respectively.
SNTL as a cathode.— When cycled between 2 V and 4.2 V at a rate
of C/50 (1.8 mA/g), SNTL had a sloping voltage profile, as shown in
Figure 2a, suggesting a solid-solution behavior. This is further substantiated by ex-situ XRD, in Figure 2b, where no additional phases were
identified upon sodium removal at 40% and 80% of state-of-charge
(SOC). The large irreversible capacity observed in the first few cycle
might be due to parasitic side-reactions such as aluminum passivation
and/or electrolyte degradation. After the first charge, good reversibility at an average voltage of ∼3.7 V and ∼90 mAh/g (x from 2/3 to 1/3)
was obtained for 5 cycles. The high operating-voltage coupled with
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ECS Electrochemistry Letters, 3 (4) A23-A25 (2014)
θ
(a)
(b)
Figure 1. (a) XRD of SNTL powders prepared from nano-precursors fired at
900◦ C at different time duration (bottom three) and prepared from micronsized precursors fired at 900◦ C for 12 h, (b). Morphology of SNTL powders
prepared from micron- and nano-sized precursors.
almost a sloping voltage profile make this material attractive for developing high-voltage Na-ion batteries. To obtain more useful capacity,
i.e. reduce x below 1/3, the higher cutoff voltage was increased to
4.5 V. But this led to a significant capacity fade as seen in Figure 2c.
At the high-voltage charge, there was a sign of a plateau indicative
of a two-phase behavior, as seen in Figure 2c at around 4.25 V of the
second cycle. Ex-situ XRD characterization of the film, after being
charged to 4.5 V, confirmed the presence of an unidentified phase
possibly related to O2-type phase, shown in Figure 2d. This trend
is similar to another P2-type material Nax Ni1/3 Mn2/3 O2 12,13 wherein
operating in the P2-O2 regime leads to capacity decay.
The rate performance of powders from nano-precursors and
micron-precursors were displayed in Figure 2e. For powders from
nano-precursors the capacity drops to 80%, 50% and 8% of the initial
value (C/20 rate) at C/5, 1C and 2C rates respectively. They perform
slightly better than those prepared from micron-sized powders due to
their smaller particle sizes and higher surface area. We expect we can
further improve the rate performance by reducing the particle sizes
through wet-chemical methods. The cycling test at C/10 rate between
(c)
(d)
Figure 3. (a) Galvanostatic charge/discharge at C/20 rate. (b) Ex-situ XRD
of SNTL films after being reduced to 0.2 V in the first cycle and C/10 for
25 cycles. (c) Rate study of SNTL anode between 0.2 and 2 V. (f) Cycling
study of SNTL anode between 0.2 and 2 V at C/5 rate.
2 and 4.2 V was shown in Figure 2f. The initial few cycles involves
parasitic side-reactions such as aluminum passivation and/or electrolyte degradation and this contributes to low coulombic efficiency.
However, the efficiency improves upon subsequent cycling and the
capacity values remained fairly stable, after the initial passivation.
SNTL as an anode.— When cycled between 0.2 V and 3 V at a
rate of C/20 (4.5 mA/g), SNTL again had an overall sloping voltage
profile, as shown in Figure 3a. The relatively steep voltage change at
θ
(a)
(b)
(c)
(d)
(e)
(f)
θ
Figure 2. (a) Galvanostatic charge/discharge at C/50 rate with 4.2 V cutoff voltage. (b) Ex-situ XRD of SNTL films (with 4.2 V cutoff) at the pristine state,
40% state-of-charge (SOC), and 80% SOC. (c) Galvanostatic charge/discharge at C/50 rate with 4.5 V cutoff voltage. (d) Ex-situ XRD of SNTL films after being
charged to 4.5 V. (e) Rate study of SNTL cathode prepared from nano- and micron-sized precursors. (f) Cycling study of SNTL cathode at C/10 rate.
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ECS Electrochemistry Letters, 3 (4) A23-A25 (2014)
x ∼ 1 is probably due to the slow diffusion when Na site becomes fully
occupied. There was also a first cycle irreversible loss. Subsequent
cycling has an average voltage of ∼ 0.7 V and capacity of ∼ 75 mAh/g.
Ex-situ XRD experiment on SNTL films reduced to 0.2 V in the
first cycle (Figure 3b) indicates that the material retained its original
structure upon sodium insertion. This confirms that the reaction is
intercalation-type and not a conversion reaction with reduction of
transition metals to their metallic state. The low redox potential of
Ti4+ /Ti3+ (∼0.7 V) coupled with the high redox potential of Ni2+ /Ni3+
(∼3.7 V) in SNTL provides the opportunity to fabricate 3 V sodium
ion batteries based on a single material. This can greatly simplify
electrode design and reduce manufacturing costs since a single powder
preparation and casting process can be used to produce the electrode
film that can be used both for cathodes and anodes.
SNTL performed better at high currents in the anodic region
(Figure 3c), compared with in the cathodic region (Figure 2e).
Capacity values of 75 mAh/g, 73 mAh/g, 70 mAh/g, 53 mAh/g,
43 mAh/g and 30 mAh/g were obtained at C/20, C/10, C/5, 1C,
2C and 4C rates, respectively. The improved rate performance in the
low potential region might be due to the presence of larger number
of titanium ions, i.e. 2/3 of Ti vs 1/3 of Ni, forming a percolating
network leading to better electronic conduction in the transition metal
layer. Cycling studies, shown in Figure 3d, revealed that there was
a continuous capacity fade. Our ex-situ XRD on the film cycled at
C/10 for 25 cycles, in Figure 3b, suggested that the capacity fade was
not due to the formation of any secondary phases. Similar fade was
reported in another sodium titanate Na2 Ti3 O7 and it was attributed to
the interaction of sodium ions with carbon additives in the composite
electrode.20 In addition, formation of a stable solid electrolyte interface (SEI) is critical for obtaining stable capacity values, especially
for anode materials. Electrolyte reduction is often favored at such low
electrode potentials and hence needs to be kinetically hindered. We
suspect that the capacity fade might also be due to formation of an
unstable SEI. Investigating new, stable electrolyte systems or developing compatible solid electrolyte systems with good conductivity
could potentially alleviate these issues.
Conclusions
For the first time, we have studied the electrochemical properties of Nax (Ni2+ )1/3 (Ti4+ )2/3 O2 (x = 2/3) and demonstrated its
A25
potential as “bi-functional” electrode materials, i.e. either a cathode
or anode, in non-aqueous Na-ion batteries. Electrochemical testing
shows that activation of Ni2+ /Ni3+ redox couple with a range of
x = 2/3–1/3 contributes to a high operating voltage of 3.7 V. The
reversible capacities at C/50 and C/20 are 90 mAh/g and 75 mAh/g,
respectively. On the other hand, activation of Ti4+ /Ti3+ redox couple
with a range of x = 2/3–1 contributes to a low operating voltage of
0.7 V with 75 mAh/g at C/20(anode). The rate performance is better in
the anodic region probably due to the larger amount of titanium metal
ions. We have observed a good cycling performance for the cathode
and a capacity fade for the anode, probably due to interaction of Na
ions with carbon and/or unstable SEI. While further work is needed
to optimize its performance, this material holds great potential for
3 V Na-ion batteries in which cathode and anode are based on the
same material.
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